The present invention relates to endodontic files, which are employed in the preparation of tooth canals during endodontic procedures.
Endodontic procedures, which are intended to treat conditions affecting the root canals and roots of teeth, include the removal of diseased and non-vital soft tissue from the canals of a tooth, cleaning and shaping the canals, and finally filling the canals as completely as possible with a suitable filling material. The tooth canals must be shaped and enlarged in order, firstly, to ensure that all root material has been removed and, secondly, to assure that filling material can be introduced into each canal up to the tooth apex.
In such procedures, the, or each, canal of a tooth must first be exposed, or opened, by removing material, including pulp, from the tooth crown. Then a succession of files is employed in each canal in order to perform the required cleaning and shaping operations.
In endodontics, a tooth canal is generally conceptualized as being divided into thirds, these being an upper third which is closest to the tooth crown, an apical third which is closest to the tooth apex and a middle third which extends between the upper and apical thirds. As a general rule, the canal should be shaped so that the upper and middle thirds taper from the crown to the apical third, thereby providing a passage for introduction of the thinnest files into the apical third.
It is known that the desirable properties of endodontic files include flexibility, efficiency, the ability to properly cut the canal wall even over curved canal portions, and the ability to remove debris from the canal.
In order to satisfy the requirements of flexibility, it is generally considered desirable that a file be capable of following high degrees of curvature. Various file designs are adaptable to varying degrees of curvature and special files are normally employed when a canal which is highly curved is encountered.
The requirement for efficiency exists because of the need to cut the canal walls, which are made essentially of dentin, in a reasonably short period of time.
In order to be able to shape a curved portion of a canal, the cutting efficiency of the file should be higher against the wall which defines the convex side of the curved path than against the wall defining the concave side thereof.
As regards debris removal, it is essential that debris created during shaping of a canal be removed from the tooth prior to filling. This is achieved by giving the file a greater efficiency when being pulled than when being pushed. Thus, the greater efficiency exists when the file is being moved in a direction out of the canal.
The basic approach to increasing flexibility is to reduce the cross section of the file, which will be determined by the starting diameter of the stock employed to form the file and the form in which the file flutes are ground. As the depth of the flutes increases, the cross section of the file decreases and the instrument becomes more flexible. In addition, the sharper the cutting edge of the file the less metal will be present in its cross section. Of course, the cross section of a file cannot be reduced indefinitely since a smaller cross section reduces the ability of the file to withstand axial stresses, and thus the danger of the distal end of the file becoming broken in a tooth canal.
The efficiency of a file essentially relates to the angle of the cutting edge, or the cutting angle. It is generally considered that a file is most efficient with a cutting angle of 0.degree. to 22.5.degree., the angle being that which exists between the surface which defines the leading edge of the blade and a plane normal to the axis of the file. The leading edge of the blade is that which faces the blade handle since the cutting action is to be effected by pulling motions. Endodontic files may have a negative cutting angle, which will be less efficient, but is employed under certain circumstances.
During the course of preparation of a tooth canal, a number of different types of files may be called upon to perform different functions. Two types of files which are widely used in these procedures are those known as k files and the Hedstrom files.
The k file is characterized by a relatively short pitch, a small cross-sectional area and a cutting angle in the neighborhood of 0.degree., while a Hedstrom file is characterized by a larger pitch, a larger cross-sectional area and a larger positive cutting angle. Because of the different cutting properties possessed by files of these types, it is generally considered desirable to utilize a k file to enlarge and shape the apical third of a canal and Hedstrom files to perform this function with respect to the upper and middle thirds.
It is also known that the configuration of the tip of an endodontic file can have a decisive effect on the preparation of curved canals. During the cutting of such a canal, if the cutting edge in the vicinity of the tip is relatively aggressive with respect to the convex side of the canal, the tip will be urged toward the center of the canal. However, such a tip can give rise to a condition known as zipping. A tip with a small angle and a rounded end can alleviate this problem.
In a typical endodontic procedure, after a canal has been opened, Hedstrom files will be employed to shape and enlarge the upper and middle thirds, after which recourse is had to k files to prepare the apical third.